fat
'n'
pretty

Sound
Clips

Guitar

flash req'd

flash req'd

The
Fat 'n' Pretty is a two channel optical
compressor/overdrive that
sounds great. It's based on a circuit that Johan Blomdahl
dubbed the
LA-Light, because he designed to mimick the functional
blocks of the
well known LA2A Leveling Amplifier (compressor). Johan's
design is best
suited to line level input signals, and he mostly uses it
for vocals.
Here is what Johan's original design looks like:LA-Light schematic

My goal was to modify the circuit to make it useful for
low level
signals (bass, guitar, etc.) while retaining the original
functionality
for line level inputs. I also wanted to minimize the
amount of wiring
to keep the assembly as simple as possible. So, I designed
a layout
that incorporates PCB mounted potentiometers.

Along the way, Johan pointed out that the LEDs in the
sidechain were in
a typical clipping orientation, and suggested the
posibillity of
getting a second, distorted output from the LEDs. Well, of
course, he
was right. So, after some tinkering, re-arranging, and
modifying, this
is what I came up with:

The second stompswitch selects between either the
compressor (Fat)
signal, or the overdrive (Pretty) signal for the main
output. There are
two pannel mounted bi-color LEDs. One is the on/off status
indicator.
The other is the threshold indicator, indicating both the
amount of
signal attenuation in the (Fat) compressor section, and
the degree of
clipping that is occurring in the sidechain (Pretty)
overdrive section.
When the Fat compressor output is selected, both LEDs are
green and the
green knob sets the output level. When the Pretty
overdrive output is
selected, both LEDs are red and the red knob sets the
output level. The
two knobs can be set so that the output level sounds the
same when
switching between Fat and Pretty, or so that there is a
boost when
switching from one channel to the other.

The Fat 'n' Pretty has a variable gain input buffer that
allows the
response, sensitivity to be adjusted to suit different
level inputs
such as line/low level or instruments with active/passive
electronics.
There is also enough gain to overdrive the circuit in
either Fat or
Pretty mode. The threshold and ratio controls, which do
what you'd
expect them to do for a compressor, allow a very wide
range of
different character overdrive sounds to be produced. It's
lots of fun
to play with.

The Fat compressor output is both very tweakable and
transparent at the
same time. It does warm up the input a bit, in a very nice
way. But,
there is a range of tame compression settings that you
might not notice
until you turn it off. At the other end of the spectrum,
it'll
certainly do some pumping and breathing if you want to
take it over the
edge.

The Brite/Warm switch that shunts one of the capacitors in
the output
stage doesn't seem to have much effect on the sound.
Although, it
sounded good on my breadboard, I'm not sure why. I really
like
everything else about this pedal a lot. So, for now, I'm
not going to
mess with it.

This circuit requires a bi-polar power supply. I wanted to
have a lot
of headroom, so I went with +/-15V. The power supply
circuit I used is
shown on the schematic. I use a 18VDC wall wart to power
it. If you
build this power supply, make sure to keep the power
ground separate
from the circuit ground, and put the regulator on a
heatsink.